Stabilisation of paper and cardboard against dimensional change



United States Patent "ice 3,230,108 STABILISATION OF PAPER AND CARDBOARD AGAINST DIMENSIONAL CHANGE Bruno Stefan, Vladimir Marek, Emmenbrucke, Switzerland, assignor to Societe de la Viscose Suisse, Emmeribrucke, Lucerne, Switzerland, a Swiss body corporate No Drawing. Filed Nov. 16, 1962, Ser. No. 238,272 Claims priority, applicationGreat Britain, Nov. 24, 1961, 42,178/61 9 Claims. (Cl. 117-154) This invention relates to dimensionally stabilised paper and to methods of its production. made from cellulose fibres expands and contracts with variations in the relative humidity of the atmosphere.

Although thesedimensional changes do notamount to much, there are a number of applications Where dimensional stabilisation of paper-and cardboard is of considerable importance. This is the case, for example, when paper is used for multicolour printing, for maps, and, especially, for tabulating punch cards, where dimensional changes may lead to inconvenient misregistrations.

The dimensional. instability of paper is mainly caused by the swelling and shrinking of the cellulose fibres at varying humidities. However, as it seems that wet paper shows. a greater dimensional increase than is possible'as a result offibre swelling alone, it is also probable that some fibre separation occurs due to the breaking of interfibre bonds. Furthermore, contact with moisture produces a relaxation of stresses present in dry paper, and such relaxation also results in changes of volume. The dimensional changes of paper are, therefore, not simply reversible, and, when paper is exposed to varying atmospheric conditions with the same humidity at the beginning and at the end of the cycle, the dimensions ofthe paper are usually not the same. mensionalchanges is also dependent on the degree of beating of the'cellulose pulp used and on the manufacturing conditions of the paper. I

Various-methods have been proposed to suppress the changes occurring in and between the fibres and to produce paper of good dimensional stability. Paper has been heated in the presence of acids or metal salts, thermosetting resins of the phenol-formaldehyde and urea-formaldehyde type have been added, and polyhydric alcohols and esters of'polycarboxylic acids have been used. It has also been" tried to prevent the swelling of paper by a treatment with formaldehyde vapour in the presence of small amounts of alum, zinc chloride, or of formic or hydrochloric acids. This treatment requires, however, rather elecated temperatures, usually 120 C, and although the paper produced has a good dimensional stability and wet strength, it is very brittle and suffers a considerable decrease of its mechanical strength, especially of its folding endurance and elongation at break.

It'is an object of the present invention to provide a method for the manufacture of paper and cardboard which possesses a high wet strength, a substantial dimensional stability against expansion and contraction with changes in atmospheric humidity, and good mechanical properties in the dry state.

This object is accomplished by the present invention which-provides a method or the treatment of paper and cardboard consisting essentially of natural and/or regenerated cellulose fibres and possessing a high wet strength and a substantial dimensional stability While retaining or even improving its mechanical properties in the dry state, which comprises impregnating paper or card-' board consisting essentially of natural and/ or regenerated cellulose fibres, with a liquid composition containing formaldehyde, a halogen-containing acid, water, and a water- It is known that paper The amount of the dimiscible solvent. Although hydrobromic acid has proved to give the highest wet strength, hydrochloric acid is the.

acetic acid is preferred as a cheap and suitable medium.

The best results have been obtained by the use of compositions containing between 1 and 5 percent of formaldehyde, between 1 and 5 percent of hydrochloric acid,-

water and at least 60 percent of acetic acid, all figures in percent meaning percent by weight of the total Weight of the liquid composition. As shown by the examples below very good results are obtained with a water content of at most 15%.

The method of treatment according to the present invention is very simple and easy to perform. Preferably, the paper is immersed during 5 to 60 minutes in a bath containing the liquid formaldehyde composition, washed with faintly alkaline water of pH=8, and heatdried. The optimum period of immersion depends on the cencentration of the reagentsand on the temperature of the bath, which is usually kept between 20 and 40 C. Actually, the paper need not remain in the bath during the whole time of the reaction. pregnate the material thoroughly for a few seconds, to squeeze out the superfluous liquid, and to keep the paper wetted with the liquid as long as required before wash ing. On a technical scale, the process can be carried out discontinuously or it may be carried out in a continuous manner by feeding paper from a supply roll through the impregnating bath, taking it up again on a further roll, keeping the rolls soaked during the prescribed time,

passing the paper through an aqueous Washing bath, and

drying the treated paper on the heated cylinders of a paper machine It is remarkable that paper treated according to the present invention not only acquires a high wet strength and a very good dimensional stability, but also acquires an increased dry strength and a considerably higher folding endurance, while its elongation at break'is practically no change of its elongation at break. When the stabilised I paper is exposed to a humidity change from to 100% and again to 65%, the improvement in dimensional stability as compared with untreated paper is 90% in the machine direction and over in the cross-machine direction. In a cycle involving humidities of 65, 90, 65, 10 and 65%, the improvement in dimensional stability in the cross-machine direction even reached 94%.

It has already been mentioned that the magnitude of dimensional changes also depends on the degree of heating of the cellulose pulp used, and it is known that paper of a'finefibre structure is especially sensitive to humidity changes. In this connection it may be noted that paper made from a cellulose pulp beaten during'90 minutes and treated according to the present invention still has a wet breaking length of over 2000- metres and an improved dimensional stability of 55% as compared with an untreated sample.

The increase of the folding endurance of paper stabilised with a liquid formaldehyde composition according to the invention is usually more than twice that of an Patented Jan. 18, 1966 It is sufficient to im untreated sample. This is in sharp contrast to the above mentioned stabilisation method using formaldehyde in the vapour phase with subsequent heating, which causes the folding endurance of the paper to drop considerably.

The quantity of water contained in the liquid formaldehyde composition has an important influence on the results of the treatment and this factor can easily be used to regulate the desired stabilisation effect. Thus, two samples of the same paper treated under otherwise equal 4 Example 11 Three different kinds of paper leaves each having a weight of 100 g./m. are made from bleached sulphate cellulose fibres, the first one from unbeaten material and the second and third leaf from cellulose beaten in a Valley beater with a lbs. load during 30 and 90 minutes respectively. From all three kinds of leaves samples are left untreated, while other samples are stabilised with a liquid formaldehyde composition in the manner described conditions with liquifi compositions cqntaining 10 and 10 in Example I. Tests for Wet strength and dimensional f z g Showed relative wet strengths stability of the various leaves have the following resultsz 0- The following examples will serve to illustrate the invention. Figures in percent as regards componeifits off the Percent 1 liquid compositions used mean percent by weig t o the 15 s o total Weight of the liquid compositions. Dry and wet Cellulose Paper g fi ifiifggfijfi fi tensile strength (expressed as breaking length) and elon- 100%, 65%. gation at break are tested by methods known per se. Dimensional changes of the paper are determined after exposure to various cycles of relative humidities by Unbeaten 716 0,20 measuring the length of paper strips to within 0.1 mm. 30 min 3 g-'18 and are expressed in percent of the original length of Untreated .1: the strips. The improvement I in dimensional stability 2y &2 :538 in percent is then: i

Ch f t t d paper =Dried over phosphorus pentoxide. =Too small to be measured. Change of stabilised paper X100 Change of untreated paper Example III Unless otherwise stated, testing figures refer to tests in p the machine direction of the paper (R.H.=Relative Hu- Paper leaves made from equal.parts natural and midity, M.D.=Machine Direction, C.M.D.=Cross-Magenefated celluioseifibres i having a Fi of 37 chine Direction) are immersed in different llquid compositions all of them Example I having a temperature of 20 C. and each containing 3% of formaldehyde, 88.5% of acetic acid, and 7.5% of Paper Sheets made from equal Paris of natural and 3r water. In addition, the first composition contains 1% generated cellulose fibres and having a Weight of 45 a of hydrobromic acid, the second composition 1% of hyare immersed during 20 minutes in a liquid COITI- drochloric acid, and the third composition 1% of perchlo position having a temperature of 28 C. and nta g ric acid. A fourth composition serving as control is made of formaldehyde, of hydrochloric acid, 88.5% without an additional acid and contains instead 1% more of acetic acid, and 7.5% of Water. The paper is then 40 f t washed with faintly alkaline water f pH=8 and i d at After immersion during 10 seconds the paper leaves are 120 C. The testing es s for P p stabilised in this squeezed out and left impregnated with their respective manner and for an untreated control sample ar Sh Wn liquid composition during 30 minutes at room temperain the following table: ture, When they are washed with faintly alkaline water. The following table shows the wet strength of the four Percent Dimen- P p leaves: Breaking length sional change after (m) Elonga- 65%,100%, 65% Bredklng length Paper 253;? cycle Acid used in liquid composition wet (m) (percent) Hydrobromic acid 962 Hydrochloric acid 763 Dry Wet C'M'D' Perchloric acid 490 st l 3,673 1, 973 2.9 +0. 04 +0.13 Without additional acid below Untreated 2,685 130 2.9 0.33 0.49

Example IV Thus the stabilised paper possesses a dry strength increased by 40%, a wet strength increased 15 times, and Porous P p leaves made from equal Parts Of {latllrai an improvement I in dimensional stability of 90% in the and regenerate? cellulose fibfes and hflvmg Weight of machine direction and of 70% in the cross-machine dii 8- immersed dul'lng 3o.mlnllies 111 diifefent rection, as compared with untreated papen liquid compositions all of them hav1ng a temperature of Exposure of the same paper to another humidity cycle C and eafih cfmialnlng 23% of formaldehyde, gives the following results; ofhydrochloric acid, between 55.8% and 88.3% of acetic acid, and between 7.5 and 40% of water. The paper Percent Dimensionalchange leaves are then washed with faintly alkaline water and after 90%, 65%, 10%, dried at 120 C. The testing results of the leaves are as Paper 65% R.H.-cycle 65 follows:

M.D. O.M.D.

Water con- Breaking length (m) Relative wet Elongation tent otllquid strength at break stabilised 1 1 composition (percent) (percent) Untreated 1 6 (percent) Dry Wet Thus the improvement I in dimensional stability of the Z g? is stabilised paper as compared with untreated paper is 90% 1:11: 2:300 33 in the machine direction and 94% in the cross-machine 2,190 Below 200 Below 10 Example V Porous unsized paper leaves made from equal parts of natural and regenerated cellulose fibres and having a weight of 45 g./m. are immersed during 5 seconds in a liquid composition having a temperature of 20 C. and containing 4.2% of formaldehyde, 2% of hydrochloric acid, 83.1% of acetic acid, and 10.7% of water. The leaves are squeezed out and kept impregnated at room temperature for different periods, when they are washed with faintly alkaline water and dried at 120 C. The following table shows the wet strength and folding endurance of the leaves in relation to the period of impregnation and in comparison with an untreated control leaf:

I claim:

1. A process for dimensionally stabilizing a paper product consisting essentially of cellulosic fibers selected from the class consisting of natural and regenerated cellulose, comprising impregnating the paper product with a liquid composition which comprises, in weight percent, 15% formaldehyde, 15% of a halogen-containing acid and at least 60% of a water-miscible liquid with the balance essentially water, washing the impregnated paper product with faintly-alkaline water and subsequently drying the product whereby said product is dimensionally stabilized.

2. A process as claimed in claim 1, in which the watermiscible liquid is acetic acid.

3. A process as claimed in claim 1, in which the paper product is kept in contact with the liquid composition at a temperature in the range 20 C. to C.

4. A process as claimed in claim 2, in which the liquid composition comprises 1-5% of formaldehyde, 1-5% of halogen-containing acid, and at least of acetic acid, the balance being water.

5. A process as claimed in claim 2, in which the liquid composition contains at most 15% of water.

6. A process as claimed in claim 5, in which the halogen-containing acid is hydrochloric acid.

7. A process as claimed in claim 5, in which the halogen-containing acid is hydrobromic acid.

8. A process as claimed in claim 5, in which the halogen-containing acid is perchloric acid.

9. A process for dimensionally stabilized paper consisting essentially of cellulosic fibres selected from the class consisting of natural and regenerated cellulose, which comprises impregnating the paper at a temperature in the range of 20 C. to 40 C. with a liquid composition comprising about 3% of formaldehyde, about 1% of hydrochloric acid, about 88.5% of acetic acid, the balance being water; washing the impregnated paper with faintlyalkaline water having a pH value of about 8 and drying the washed paper at a temperature about C. whereby said paper is dimensionally stabilized.

References Cited by the Examiner UNITED STATES PATENTS 1,816,973 8/1931 Kantorowicz 117154 XR 2,903,328 9/1959 Kress 8116.4 XR 3,046,079 7/ 1962 Reeves 8116.4

WILLIAM D. MARTIN, Primary Examiner.

RICHARD D. NEVUS, Examiner. 

1. A PROCESS FOR DIMENSIONALLY STABILIZING A PAPER PRODUCT CONSISTING ESSENTIALLY OF CELLULOSIC FIBERS SELECTED FROM THE CLASS CONSISTING OF NATURAL AND REGENERATED CELLULOSE, COMPRISING IMPREGNATING THE PAPER PRODUCT WITH A LIQUID COMPOSITION WHICH COMPRISES, IN WEIGHT PERCENT, 1-5% FORMALDEHYDE, 1-5% OF A HALOGEN-CONTAINING ACID AND AT LEAST 60% OF A WATER-MISCIBLE LIQUID WITH THE BALANCE ESSENTIALLY WATER, WASHING THE IMPREGNATED PAPER PRODUCT WITH FAINTLY-ALKALINE WATER AND SUBSEQUENTLY DRYING THE PRODUCT WHEREBY SAID PRODUCT IS DIMENSIONALLY STABILIZED. 